The present invention relates to circuit breakers and, more particularly, to circuit breakers including thermal and magnetic trip units.
Circuit breakers typically provide instantaneous, short time, and long-time protection against high currents produced by various conditions such as short-circuits, ground faults, overloads, etc. In a circuit breaker, a trip unit is the device that senses current (or other electrical condition) in the protected circuit and responds to high current conditions by tripping (unlatching) the circuit breaker""s operating mechanism, which in turn separates the circuit breaker""s main current-carrying contacts to stop the flow of electrical current to the protected circuit. Such trip units are required to meet certain standards, e.g., UL/ANSI/IEC, which define trip time curves specifying under what conditions a trip must occur, i.e., short time, long time, instantaneous, or ground fault, all of which are well known.
One type of trip unit is known as a thermal and magnetic trip unit. A thermal and magnetic trip unit includes a magnetic assembly and a thermal assembly. The thermal assembly typically includes a bimetallic element through which electrical current flows. As current flows through the bimetallic element, the bimetallic element heats up and bends due to the different coefficients of expansion in the metals used to form the bimetallic element. If the temperature rise is sufficient, the bimetallic element bends enough to move an associated trip latch, which unlatches the operating mechanism to separate the main current-carrying contacts. The thermal assembly is typically used to sense an overload condition.
The magnetic assembly typically includes a magnet core (yoke) disposed about a current carrying strap, an armature (lever) pivotally disposed near the core, and a spring arranged to bias the armature away from the magnet core. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet core. The magnetic field acts to rapidly draw the armature towards the magnet core, against the bias of the spring. As the armature moves towards the core, the end of the armature moves an associated trip latch, which unlatches the operating mechanism causing the main current-carrying contacts to separate.
Thermal and magnetic trip units must be calibrated to ensure that the circuit breaker trips at the appropriate current conditions. Calibration typically includes adjusting a distance between the bimetal and its associated trip latch and between the armature and its associated trip latch. However, establishing and maintaining calibration can be made difficult due to relative motion of the operating mechanism and the trip unit.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a circuit breaker for a multi-pole electrical distribution circuit, the circuit breaker including a cassette associated with each pole in the multi-pole electrical distribution circuit. Each cassette includes a housing, a pair of electrical contacts disposed in the housing, and a thermal and magnetic trip unit supported by the housing. The thermal and magnetic trip unit initiates separation of the pair of electrical contacts in response to an overcurrent condition in the multi-pole electrical distribution circuit.
In one embodiment, the housing includes a first compartment having the pair of electrical contacts disposed therein and a second compartment having at least a portion of the thermal and magnetic trip unit disposed therein. The first and second compartments have a wall disposed therebetween for isolating the second compartment from gasses generated by separation of the pair of electrical contacts. The first compartment may be in fluid communication with an exterior portion of the housing through a duct adjacent the second compartment, such that the gasses pass through the channel to the exterior portion of the housing.
In another embodiment, an end of the housing includes a pair of opposing slots formed therein for receiving edges of a load terminal. The edges of the load terminal may each include a detent formed thereon for retaining the edges within the pair of opposing slots.
Another embodiment includes a lever having a first end disposed proximate an end of a bimetallic element and a second end disposed proximate an armature in the magnetic assembly of the trip unit. At least one of the bimetallic element and the armature rotate the trip lever to unlatch an operating mechanism. The lever and the bimetallic element may extend into the second compartment through an opening in the top of the housing.